1. Synchronization

2. Clock Synchronization In a centralized system time is unambiguous. In a distributed system agreement on time is not obvious. When each machine has its own clock, an event that occurred after another event may be assigned an earlier time.

3. Clock Synchronization Algorithms The relation between clock time and UTC when clocks tick at different rates. If 1 –  dC/dt < 1 +  maximum drift rate Resynchronization interval  maximum time difference

4. Cristian's Algorithm synchronization with a time server Time must never run backward gradual changes Non zero message propagation time

5. The Berkeley Algorithm synchronization without time server a)The machines answer b)The time daemon asks all the other machines for their clock values c)The time daemon tells everyone how to adjust their clock no Universal Coordinated Time available

6. Centralized algorithms have disadvantages. Decentralized algorithms can use averaging methods. NTP (Network Time Protocol) provides an accuracy of 1-50 msec using advanced algorithms. For many purposes it is sufficient that all machines agree on the same time. Logical clocks

7. Lamport Timestamps If a b C(a) < C(b) If a sen., b rec. C(a) < C(b) C(a)  C(b) If C(a) < C(b) C(a) = C(b)+1 We obtain a total ordering of all events in the system Lamport timestamps don’t capture causality Vector timestamps

8. Global State it is the local state of each process, together with the messages currently in transit a distributed snapshot reflects a consistent global state

9. Global State Organization of a process and channels for a distributed snapshot a)A process P sends a marker along each of its outgoing channels any process P may initiate the algorithm recording its local state.

10. Global State b)Process Q receives a marker for the first time and records its local state c)Q records all incoming message d)Q receives a marker for its incoming channel and finishes recording the state of the incoming channel A process has finished its part of the algorithm when it has received a marker along each of its incoming channels, and processed each one. Many snapshots may be in progress at the same time.

11. The Bully Algorithm The bully election algorithm a)Process 4 holds an election b)Process 5 and 6 respond, telling 4 to stop c)Now 5 and 6 each hold an election Selecting a coordinator

12. d)Process 6 tells 5 to stop e)Process 6 wins and tells everyone

13. Mutual Exclusion: critical regions in distributed systems A Centralized Algorithm a)Process 1 asks the coordinator for permission to enter a critical region. Permission is granted b)Process 2 then asks permission to enter the same critical region. The coordinator does not reply. c)When process 1 exits the critical region, it tells the coordinator, which then replies to 2

14. A Distributed Algorithm requires a total ordering of all events in the system a)Two processes (0,2) want to enter the same critical region at the same moment. b)Process 0 has the lowest timestamp, so it wins. c)When process 0 is done, it sends an OK also, so 2 can now enter the critical region. This algorithm is worse than the centralized one, but possible

15. A Toke Ring Algorithm a)An unordered group of processes on a network. b)A logical ring constructed in software. start

16. Comparison A comparison of three mutual exclusion algorithms. Algorithm Messages per entry/exit Delay before entry (in message times) Problems Centralized32Coordinator crash Distributed2 ( n – 1 ) Crash of any process Token ring 1 to  0 to n – 1 Lost token, process crash

17. The Transaction Model A transaction permits that a whole set of related instructions would be successfully completed or none would be completed. Special primitives are requested Examples of primitives for transactions. PrimitiveDescription BEGIN_TRANSACTIONMake the start of a transaction END_TRANSACTIONTerminate the transaction and try to commit ABORT_TRANSACTIONKill the transaction and restore the old values READRead data from a file, a table, or otherwise WRITEWrite data to a file, a table, or otherwise

18. The Transaction Model Transactions are ACID: Atomic : to outside, they happen indivisibly Consistent : they don’t violate system invariants Isolated : concurrent transactions don’t interfere Durable : the changes are permanent

19. Distributed Transactions a)A nested transaction (it follows a logical division of the work) b)A distributed transaction (it is logically flat and operates on distributed data)

20. Private Workspace a)The file index and disk blocks for a three-block file b)The situation after a transaction has modified block 0 and appended block 3 c)After committing

21. Writeahead Log a) A transaction b) – d) The log before each statement is executed In distributed systems each machine keeps its own log of changes to its local file system x = 0; y = 0; BEGIN_TRANSACTION; x = x + 1; y = y + 2 x = y * y; END_TRANSACTION; (a) Log [x = 0 / 1] (b) Log [x = 0 / 1] [y = 0/2] (c) Log [x = 0 / 1] [y = 0/2] [x = 1/4] (d) rollback

22. Concurrency Control allows several transaction to be executed simultaneously leaving data in a consistent way General organization of managers for handling transactions.

23. Concurrency Control General organization of managers for handling distributed transactions.